The invention relates to a method for unambiguous determination of a range to and/or of a relative velocity of an object with respect to a motor vehicle by means of a frequency-modulation continuous-wave radar in the motor vehicle. The frequency-modulation continuous-wave radar transmits a predetermined sequence of frequency-modulated signal pulses (Chirps) in an individual measurement cycle. An unambiguity area for the determination of the range and/or an unambiguity area for the determination of the relative velocity are/is determined by the sequence of frequency-modulated signal pulses. The invention also relates to a driver assistance device and to a vehicle having a driver assistance device such as this.
The present interest therefore relates to a frequency-modulation continuous-wave radar (frequency modulated continuous wave radar). This is referred to in the following text as an FMCW radar, or simply as a radar. A radar such as this operates as follows: the radar transmits a predetermined number of frequency-modulated signal pulses (also referred to by the term “Chirps”) as a transmitted signal in a measurement cycle. The radar then receives a received signal, which is the transmitted signal reflected by an object. The transmitted signal is compared with the received signal. Two major measurement variables of the radar are the range and the relative velocity. The range is determined by the delay time of the transmitted signal; the relative velocity is determined by the change in the frequency of the transmitted signal, resulting from the Doppler effect.
The use of radars such as these in motor vehicles is already known from the prior art. The document DE 10 2005 048 209 A1 describes a motor vehicle having an FMCW radar such as this. This method is distinguished in that only the range but not the relative velocity is determined in predetermined time periods within a measurement cycle, for objects in at least one subarea of the area around the motor vehicle. A sequence of 16 to 64 frequency-modulated signal pulses, each having a time duration of about 250 μs, is transmitted by the radar in one measurement cycle and per radar lobe (per beam); the received signal for the entire sequence of signal pulses is then evaluated coherently. This allows a relatively high Doppler-frequency resolution and a relatively high signal-to-noise ratio (SNR), while at the same time making good use of the available measurement time. The parameters of the transmitted signal and the system parameters were chosen such that good measurement results could be achieved using the existing components, specifically in particular on the basis of the chosen time duration of the individual signal pulse of 250 μs, and on the basis of a sampling rate of an analogue-digital converter for the received signal. In this case, the sampling rate is 1 MHz.
In reality, the measurement of the two variables—the range and the relative velocity—is subject to limitations: the resolution and the unambiguity of the measurement are restricted. In principle, a radar can process a plurality of targets at the same time. The resolution capability of the radar indicates how far the targets must be apart from one another in order to allow the radar to detect them as two separate targets. Both range and velocity resolution are possible for targets using the radar according to document DE 2005 048 209 A1. By way of example, a range resolution of 1 m means that the range from the radar to two targets must differ by at least one 1 m in order to allow them to be resolved by the radar.
As already stated, the range measurement and the relative velocity measurement may be ambiguous. In general, this results from sampling effects. The ambiguity problem is illustrated by the following example: by way of example, a radar can detect targets up to a maximum range of 300 m. The range unambiguity (the unambiguity area) may, however, for example be 100 m, and therefore significantly below the maximum range of the radar. In this case, a measured value for the range of about 10 m means that the target may be at a range of 10 m, 110 m or 210 m away from the radar.
The choice of the parameters for the transmitted signal, as well as the system parameters, influences the unambiguity area for the determination of the range and the unambiguity area for the determination of the velocity. As stated above, in the case of the parameters mentioned, the unambiguity area for the range is about 100 m, and the unambiguity area for the determination of the velocity is about 80 km/h. However, in practice, the radar can also detect targets which are at a range of considerably more than 100 m away from the radar. In practice, in particular on motorways, relative velocities may additionally occur between −200 km/h and +200 km/h. There is therefore a major requirement to increase the unambiguity areas for the range and the velocity overall.
In order to increase the unambiguity area for determination of the range, in principle it will be possible to increase the sampling rate of the analogue/digital converter in the receiver. However, this solution is not optimal. Increasing the sampling rate of the analogue-digital converter increases the costs for this component. On the other hand, the unambiguity area for determination of the velocity could be increased by reducing the time duration of an individual frequency-modulated signal pulse. However, in this case, the sampling rate of the analogue/digital converter would then have to be correspondingly increased in order to keep the range resolution constant; in addition, correspondingly more signal pulses would have to be transmitted for the same Doppler-frequency resolution. This once again results in the problem of increased hardware costs.